DIYHiFi.org

You better save those J74s, for when you really need them.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

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Max, I think the i-input has to be held at a DC offset equal to half the Vsupply of these PCM's which I believe is 2.5V's not zero, as they run with a virtual ground not having a dual supply to them. You may want to consider the PCM1704 which doesn't have this issue and is a better chip.

Hi Glen,
I have read many comments on this dac and I don't think so that pcm1794 have 2.5v at the output. If somebody can confirm to me? thank you! Maxpou

Hi Guys,
I think to have a problem with my dc servo because it make correction only of 40mV. If I put a dc voltage at my input with 50K pot , I decrease the value for have 0.5V at output the servo drop the dc voltage at 0.465V. Why? Maxpou

Don't any of you guys know how to read a data sheet? You are confusing this chip with something like a '1738.

I don't want to have to 'splain this again...............

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

Hi jocko,
I read the datasheet, but I have seen nothing that explains it. PLease give a link where are you explain it and I will read. And For dc servo can you help me? Thank you! maxpou

Look at the data sheet for the '1738, and tell me what differences that you see. If you don't see them, you need to look harder.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

Hi Jocko,
the only difference I see are the lower current output and Vcom2 on 1738. I said that because of the IV converter I found for 1738. Maxpou

Center BPZ output?

exactly -6.2mA which you will find is a 2.5V vitual ground. Its typically half the Vcc (5V) on these sigma delta junk chips, but I gave up on them years ago so maybe someone else with better recollection answer.

Hi Guys,
today I read a post of Jocko on PCM1738: This DAC operates from +5 V only. In order to swing current in both directions, the output(s) have to be at Vcc/2. it is not at zero volts.

Any I/V has to be biased to be at Vcc/2, either from the internal Vcom pin, or an outside reference. The inputs that do not go to the chip have to be biased up to the same voltage as the output.

I have a ton of these as samples. The need to bias the I/V at Vcc/2 is one reason I never used them. (The TI rep was upset by this.) It isn't hard to do or deal with. I just chose not to. (And stuck with the old '1704 DACs.)

This part is kinda sorta a delta-sigma part, with some kludge scheme to allow folks like me to use an external I/V. As opposed to a conventional delta-sigma, that uses internal (crap) op-amps to combine the signals to get a voltage output.

Translation: If you don't have the inputs that would normally be grounded in your I/V going to Vcc/2, it probably won't work worth a crap. Jocko


I read many post here and in other place on this DAC ( 1794 ) and I see that it's very hard to ajust a IV with these topology for this DAC. I think that I will abort my project, it's too hard for me. Maxpou

Maxpou, you need to do like Jocko says. The PCM179x don't have the same problem as the PCM173x.

As far as I/V converter, Ohm's law suggests that a resistor makes an excellent one. So maybe the project is not so hard after all.

In-friggin'-credible.

Look, it is real simple:

The '173x explicitly shows the (+) inputs of the I/V op-amps going to bias source.

The other ones show it going to ground.

Now, if you can't figure it out at this point, you need to get a new hobby.

Jocko

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"Because, I hate you guys. I hate you guys so very, very much.

Yours,
General Cartman Lee"

I'm confused
Why get so many samples if you're not able or want to use them

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Rudolf Broertjes

Hi Charles,
I'm happy to see you here. I think you used a resistor as IV in your QB-9 with a excellent result I'm sure, but I worked on this topololy because I read this post of jocko in another tread here:

Only ideal if you neglect the fact that damn near every DAC is designed to work into zero ohms, because that means that it operates with zero volts at its output.

But hey........if linearity does not matter, and you like to add in noise........sure, why not?

Nope, DACs work best into zero ohms. That is why they invented current sources and current mirrors so that you can drive zero ohms, and move the current around (without any appreciable noise or distortion) and then dump it into your ideal I/V resistor.

I have not read on passive IV resistor. I see on the schematic that I found a 25 ohms to the ground, this is the IV resistor? Maxpou